defbinop.clj

(def *max-unroll-arity* 8) ; should probably be a defbinop keyword argument instead

(defmacro defbinop [name & fdecl]
  (let [var-supply (repeatedly gensym)]
    `(defn ~name
       ~@fdecl
       ~@(for [n (range 3 *max-unroll-arity*)]
           (let [vars (take n var-supply)]
             `([~@vars] ~(reduce (fn [x y] `(~name ~x ~y)) vars))))
       ~(let [vars (take *max-unroll-arity* var-supply)]
          `([~@vars & ~'more]
              (reduce ~name (concat (list ~@vars) ~'more)))))))

;; example

(binding [*max-unroll-arity* 16] ;; double the default
  (defbinop my-add
    ([] (+))
    ([x] (+ x))
    ([x y] (+ x y))))